How can I read VMS BACKUP tapes on a UNIX machine?

There is a utility called vmsbackup to do this. Earlier URLs for this
software are now invalid - the following is part of a set of FreeBSD
ports - it may or may not work on other UNIX systems.

www.ee.freebsd.org/ports/emulators.html#vmsbackup-3.0

Vbackup is a commercial product which can read and write VMS-compatible
BACKUP savesets.

www.bbc.com/vbackup.htm